The present invention relates to read circuitry of a magnetic storage system. In particular, the present invention relates to a bias generator for providing a bias current to a differential transistor pair that forms the read amplifier.
Magnetic storage systems store information by magnetizing adjacent bit positions on a recording surface of magnetic media, such as a magnetic disc in a magnetic disc drive. Within the magnetic disc drive, an actuator arm supports a magnetic head close to the disc surface for reading and writing information to the disc surface. In larger storage systems, disc drives include several discs stacked upon one another. An actuator assembly supports several actuator arms. Each arm supports a magnetic head near the recording surface of its respective disc.
The magnetic head includes an inductive coil which reads and writes information by sensing or creating a changing magnetic field. For example, in write mode, the magnetic head writes information by forcing a relatively large current through the inductive coil in a selected direction to polarize a bit position adjacent the magnetic head in a selected direction. By controlling the direction at which the adjacent bit positions are polarized, digital information may be written onto the disc surface.
In read mode, the magnetic head senses changes in polarization of adjacent bit positions. The changes in polarization induce a current in the magnetic head in a direction indicative of the direction in which the adjacent bit positions are polarized. The direction of the induced current is, therefore, representative of the data stored on the disc.
During read mode, the read circuitry monitors the direction of current flow induced in the magnetic head at the head contacts. The read circuitry typically includes a read differential transistor pair connected between a read amplifier. Each transistor in the pair includes a control terminal connected to the head contacts. Differential voltages at the head contacts are indicative of the direction of induced current in the magnetic head. These differential voltages drive the transistors of the read differential transistor pair between on and off states. The read amplifier provides an output signal that corresponds to the on and off states of the read differential pair. Therefore, the output signal of the read amplifier is representative of the data stored on the disc surface.
A bias current is frequently provided to the read differential transistor pair to set an operating point for each of the transistors of the read differential transistor pair. A bias circuit provides the bias current to the control terminals of the read differential pair. Between the bias circuit and each of the control terminals are a pair of input resistors. These input resistors are selected to have a sufficiently high resistance value to provide a high input impedance at the differential transistor pair relative to the output impedance of the magnetic head. The relatively high input impedance at the differential transistor pair tends to reduce the attenuation of the magnetic head signal voltage at the input of the differential pair.
The resistance value of the input resistors is temperature dependent as is the gain or beta characteristics of the differential transistor pair. Therefore, the bias current as well as the voltage drop across the input resistors due to the bias current varies with temperature. As a result, the operating point of the differential pair varies with temperature. This operating point variation of the differential transistor pair may be tolerable at relatively high supply voltages e.g. greater than 4.5 volts. However, for operation at supply voltages that are less than 4.5 volts the operating point variation of the differential transistor pair is a serious problem. This operating point variation can cause saturation of transistors in either a current source or the read amplifier, each of which are frequently connected to the read differential transistor pair.
There is a present need for a read amplifier that is suited for interconnection with a magnetic head that is capable of operation with a low supply voltage.